1. Field of the Invention
This invention pertains to a method of cooling and freezing organic-comprised articles which makes use of liquid cryogen and cold gases to provide an economical process for reducing the temperature of the article. The invention also pertains to the freezing system used to practice the method, which system comprises a combination of cryogenic freezer elements with mechanical freezer elements to provide cost savings efficiencies in terms of combined capital expenditures and operating expenses.
2. Background Art
The freezing of foodstuffs and biologicals requires careful consideration of the physical changes which occur in the material when it is frozen. Many biological or foodstuff materials must be frozen very rapidly to prevent the growth of damaging crystal formations which can break the cell structure of the material, resulting in destruction of the biological activity or food structure and taste characteristics. In addition, rapid production of a crust on the surface of the article being cooled or frozen prevents the transmission of fluids from the interior of the article to the surface of the article from which such fluids can be evaporated or carried off by the process environment. By maintaining the frozen crust over substantially the entire article surface while the article is brought to the desired frozen temperature throughout, loss of fluids from the interior of the article can be prevented or at least greatly reduced. Rapid freezing or frozen crust formation is frequently obtained by direct immersion of the articles to be frozen in a cryogenic liquid. However, typically the cryogenic media is too expensive to completely freeze an article solely by immersion.
In addition, as disclosed in U.S. Patent Application, Ser. No. 219,666, pending assigned to the assignee of the present invention, which application is hereby incorporated by reference, there is an advantage in limiting the depth of crust freezing which takes place upon exposure of the article to a liquid cryogen, so that thermal cracking of the article being frozen is reduced or prevented. By controlling the depth or thickness of frozen crust and the surface temperature of the article, and by maintaining control of the temperature profile of the article while bringing the article to the desired temperature throughout, a higher quality frozen article is produced.
Use of a liquid cryogen to crust freeze an article provides the advantages described above. The additional cooling necessary to bring the article to the desired frozen temperature throughout can be provided using cryogen vapor heat exchange with the article, as described in the above-referenced patent application. However, the cost of the cryogenic media used to provide the total heat removal necessary can be prohibitive for highly price competitive consumer articles such as foodstuffs. In cases where competitive price is critical, mechanical refrigeration can be used to achieve a portion of the cooling after crust freezing of the article.
A typical mechanical refrigeration system for cooling or freezing articles comprises a cooling chamber in which the article to be cooled is directly contacted with chilled gases which draw heat from the article into the chilled gases. Typically the chilled gases are recycled within the cooling chamber to take full advantage of their heat removal capability, although a portion of the chilled gases can be discarded after contact with the article to be cooled and replaced with new chilled gas makeup if desired. The heat transferred to the chilled gases must continually be removed during their recirculation, and the means of heat removal is commonly a vapor compression refrigerator or "chiller." The chiller typically comprises an evaporator, compressor, condenser, and expansion valve in that sequence. The chiller generally comprises a closed loop with a refrigerant recycled therein. At the evaporator, the refrigerant is changed from a liquid to a saturated vapor by indirect contact through a heat exchange surface with the gases to be cooled (chilled), whereby the heat content of the gases is reduced. Typical refrigerants used in the chiller include ammonia, chloro-fluorocarbons, and other FDA approved refrigerants. When refrigerants of the kinds listed above are used in typical chillers, the chilled gas temperatures generated in a typical mechanical freezer system refrigeration range from about -60.degree. F. (-51.degree. C.) to about 0.degree. F. (-18.degree. C.).
The heat transfer rates typically available from a mechanical refrigeration system are not sufficiently high to provide the desired crust freezing of an article as previously described. In addition, the cost of mechanical refrigeration equipment is high, requiring a substantial initial capital investment. Despite these disadvantages, mechanical refrigeration systems provide operational efficiency, in terms of heat content removed from the recirculated chilled gases per horsepower or kilowatt cost.
There is, then, an advantage in combining the use of cryogenic and mechanical freezing techniques to provide a high quality product at an economical cost for those applications wherein volume of articles to be processed justifies the initial capital equipment investment in the mechanical system.
An undated sales brochure, entitled: "Innovation and Efficiency in Food Freezing Equipment" by Koach Engineering and Manufacturing Inc., Sun Valley, California describes commercially available cryogenic and mechanical freezing units and recommends use of a combination of these units. The description points out that the combination is attractive due to utilization of the best features of each unit. The brochure diagram shows side-by-side immersion and mechanical units with direct flow of cold nitrogen vapor to the mechanical unit. U.S. Pat. No. 3,298,133, dated Jan. 14, 1967, to R. C. Webster et al describes a method and apparatus for cryogenic freezing of food products, using liquid nitrogen and vapors thereof. The articles travel up an incline to an area where they are sprayed with liquid nitrogen; nitrogen vapors produced in the spray area are directed down the incline to precool the articles. Use of nitrogen vapors created upon contact of liquid nitrogen with the food product to provide additional cooling of the food product provides a more economical freezing system.
U.S. Pat. No. 3,376,710, dated Apr. 9, 1968, to W. E. Hirtensteiner describes an additional cryogenic food freezing apparatus which utilizes both liquid cryogen and cryogen vapors in freezing the food.
U.S. Pat. No. 3,507,128, dated Apr. 21, 1970, to T. H. Murphy, describes a continuous method and apparatus for freezing products using a combination of mechanical and liquid gas freezing techniques. Mechanical refrigeration is used to precool the product substantially to its freezing point, followed by spray application of liquid gas to substantially freeze the product, followed by mechanical refrigeration to bring the product to its desired final temperature throughout.
U.S. Pat. No. 3,512,370, dated May 19, 1970, to T. H. Murphy describes a batch method and apparatus for freezing products which is very similar to the continuous process described in U.S. Pat. No. 3,507,128.
U.S. Pat. No. 3,805,538, dates Apr. 23, 1974, to C. F. Fritch, Jr., et al.; discloses a process for freezing individual food segments which comprises contacting the segments with a spray of liquid cryogen, followed by a refrigerated gas blast and then a second spray of liquid cryogen. The refrigerated gas comprises cryogen vapor which is cooled using a refrigeration coil which is cooled by a mechanically driven compressor, an absorption system or the like. The refrigeration coil is maintained free of ice by spraying a solution of antifreeze over the surface of the coils.
The present invention provides for crust freezing of the article to be processed, followed by mechanical means cooling of the article to the desired final temperature. The present invention provides an improvement in the utilization of cryogen vapors within the process in a manner which better takes advantage of the heat removal capabilities of such vapors.